1. Field of the Invention
The invention relates to a composite body in which a ceramic component is bonded to a metal component by internal soldering. The invention also relates to the process for producing the composite body and use thereof.
2. Description of Related Art
Ceramic is used in many industrial areas since this material is very hard, resistant to high temperatures, corrosion-resistant and electrically insulating. To be able to utilize the advantages both of metals and of ceramics, it is often necessary to produce metal/ceramic composites. Here, the mechanical, chemical and thermal stability of the bond between the ceramic and the metal becomes of key importance in deciding the industrial use of the composite.
As a result of the differences in material properties of the components to be joined, the bonding of ceramic parts to metals in principle poses great difficulties, in particular in the case of internal soldering in which a component of the one material, shaped to fit accurately, is meant to be arranged in a recess within a component of the other material and be durably and firmly fixed there by means of soldering composition under the action of heat. Owing to the differences in the material parameters of the two materials, such as modulus of elasticity, yield point R.sub.p, Poisson's ratio (.nu.) and particularly the linear coefficient of thermal expansion (.alpha.), complex joint stresses occur in the composite under the soldering conditions, in particular at the material transitions ceramic/soldering composition or soldering composition/metal or ceramic/metal.
Even with the selection of materials having expansion properties matched as far as possible, for example Al.sub.2 O.sub.3 ceramic in combination with FeNiCo alloys (.RTM.Vacon), thermally induced residual stresses still occur in the composite, which stresses cause premature failure on mechanical loading. It is assumed that the cause of this is the significantly higher coefficient of thermal expansion (.alpha.) of the silver/copper soldering composition compared with Al.sub.2 O.sub.3 and Vacon. Table 1 shows the material data at the solidification temperature of the solder (780.degree. C.) taken from the literature and manufacturer's data.
TABLE 1 ______________________________________ Material data for Vacon, Al.sub.2 O.sub.3 and Ag/Cu soldering composition at 780.degree. C. Modulus of .alpha. elasticity R.sub.p Material [.multidot.10.sup.-6 .multidot.K.sup.-1 ] [.cndot.10.sup.5 MPa] [Mpa] .nu. ______________________________________ Vacon FeNiCo alloy 8.3 1.3 100 0.3 (Material no. 1,3981) Al.sub.2 O.sub.3 ceramic 8.1 3.5 -/- 0.23 Ag/Cu eutectic hard 18.9 0.22 25 0.3 solder ______________________________________
As a result of the thermally induced residual stresses, the bonding of the materials shown in the table may in some circumstances fail even during the cooling phase, without external loading, by crack formation in the brittle ceramic in the direct proximity of the material transitions.